1. Field of the Invention
The invention relates to a method of updating velocity data in case of performing a servo control on the basis of a digital encoder signal. More particularly, the invention relates to a digital encoder control method suitable for applying to a positioning servo for controlling a feed position by a paper feed control of a recording apparatus.
2. Related Background Art
An ink jet recording apparatus is widely used as a means which is installed into a printer, a facsimile, or a copying apparatus and records an image (including characters and symbols) onto a recording medium such as paper, plastic thin sheet (OHP or the like), or the like on the basis of image data.
The ink jet recording apparatus performs the recording by emitting an ink droplet from a recording head onto the recording medium and has features such that a compact size of recording means can be easily realized, an image of a high precision can be recorded at a high speed, running costs are low, and noises are small since it is a non-impact type. The ink jet recording apparatus also has an advantage such that a color image can be easily recorded by using inks of multicolors.
As driving sources of the ink jet recording apparatus, there are a carriage motor for reciprocatively driving a carriage on which the recording head is mounted, an ASF motor for feeding the recording medium, a recovery system motor for performing a head cleaning or the like, a paper feed motor for feeding the recording medium every recording scan, and the like. Hitherto, a stepping motor has often been used as such a driving source because low costs can be easily realized, a control is simple, and the like.
The noise level of the ink jet recording apparatus is low because it is a non-impact type. However, the usage of a DC motor as such a driving source is increasing for the purpose of realizing a further silent operation or the like. In this case, an encoder is generally used for obtaining control data of the DC motor.
FIG. 6 shows a model diagram of the encoder. According to the encoder, a detector 703 detects light emitted from an LED 701 through a code wheel 702 and generates a signal. Light transmitting portions 704 and light non-transmitting portions 705 are arranged on the code wheel 702 at predetermined intervals. Photodiodes 706, 707, 708, and 709 are arranged in the detector 703 at predetermined intervals. The light detected by the photodiodes 706, 707, 708, and 709 is converted into an electric signal A 710, an electric signal *A 711, an electric signal B 712, and an electric signal *B 713, which are output. The outputted electric signals 710, 711, 712, and 713 are outputted as differential outputs, Channel A 716 and Channel B 717, by comparators 714 and 715, respectively.
FIG. 7 shows waveforms of the differential output signals. A signal which is inverted at a cross point of an electric signal A 801 and an electric signal *A 802 becomes a Channel A 803. When a velocity is constant, a duty ratio of the Channel A 803 is ideally equal to 50%. However, the duty ratio changes due to various factors. An important one of the factors is a sensitivity difference of the photodiodes.
FIG. 8 shows waveforms of the differential output signals in the case where there is a difference between the sensitivities of the photodiodes. The sensitivity of the photodiode appears as an amplitude difference of the electric signals. FIG. 8 shows a Channel A 903 in the case where an amplitude of an electric signal A 901 is smaller than that of an electric signal *A 902. It will be understood from FIG. 8 that the sensitivity difference of the photodiodes changes a duty ratio of an output signal. However, it does not exert an influence on a period of the Channel A. Because of the reasons as mentioned above, generally, the period of the output signal of the encoder has the highest precision.
Position data and velocity data are obtained as control data of the DC motor from the encoder signal. To obtain more accurate data as velocity data, generally, a one-edge sampling system for counting and employing, for example, a period in a range from a rising-up edge to another rising-up edge of an output signal is used.
However, when the velocity data is obtained by the one-edge sampling system, the velocity data is not updated unless the output signal of the encoder elapses one period. That is, the number of times of update of the velocity data is only xc2xd as compared to that of a both-edge sampling system and is only xc2xc as compared to that in case of sampling both edges of two phases, i.e. the Channel A and the Channel B.
For example, when considering a paper feed control of the ink jet recording apparatus, the paper is fed at a high speed at the beginning and a servo control is performed at a low speed from a position that is slightly before a stop position. After that, the control mode is shifted to a stop mode at a position just before a target stop position, thereby stopping the paper at the target position. In this case, the operation for stabilizing the low-speed servo control at the position which is slightly before the stop position, largely influences on a stop precision of the paper.
As mentioned above, when the paper is fed at a low speed, a change in encoder signal naturally becomes slow and an updating interval of the velocity data in the one-edge sampling system also becomes long. Therefore, a case where the velocity data obtained at a servo period is not updated from previously obtained velocity data occurs, and thus a problem such that the servo operation is not stable occurs.
If the both-edge sampling system or the like is used in order to solve the above problem, although the updating interval of the velocity data becomes short, the precision of the velocity data deteriorates due to the above-mentioned reasons, and thus a problem such that the servo operation is not stable similarly occurs.
It is an object of the invention to provide a digital encoder control method which can realize the stable servo operation of a short velocity data updating interval at a precision that is equal to that of a conventional one-edge sampling system.
Another object of the invention to provide a digital encoder control method comprising the steps of: arranging driving means; arranging frequency signal generating means for generating a pulse signal of a frequency according to a driving velocity of a driven member which is driven by the driving means; arranging edge detecting means for detecting a rising-up edge and a falling-down edge of the pulse signal; and arranging period data detecting means for counting period data indicative of a period between the edges detected by the edge detecting means, wherein each time the edges are detected by the edge detecting means, the period data of the period between the same edges as the detected edges is outputted as control velocity data of the driving means.